This invention relates to novel photosensitive compositions. More particularly, it is concerned with photosensitive compositions which are photopolymerizable by the action of actinic light and which are useful in the preparation of relief image, especially relief printing plates for flexographic printing having superior resistance to solvent type flexographic ink and mechanical properties.
Many prior art references teach various photosensitive compositions useful in the preparation of relief printing plates. The flexographic printing plates obtained from unsaturated polyester type photosensitive compositions disclosed in, for example, Japanese Pat. No. 542,045 are insuffficient in resistance to solvent type flexographic ink containing organic solvents including lower alkyl alcohols such as methyl alcohol, ethyl alcohol and isopropyl alcohol; esters such as ethyl acetate and n-butyl acetate; and ketones such as acetone and methylethyl ketone. Japanese Patent Laid Open 37521/1972 describes photosensitive compositions containing a specific block copolymer as a binder and the relief printing plates obtained therefrom having good resistance to alcohol based solvent type flexographic ink. However, these compositions which are solid would be economically disadvantageous in order to provide a variety of the photosensitive elements in accordance with their use conditions such as the thickness of flexographic printing plates selected, usually from about 0.4 mm. to 10 mm. and the printing machine employed. The flexographic printing plates obtained from the photosensitive compositions containing a polyester-polyether block polymer described in British Pat. No. 1,425,274 have improved mechanical properties and printing durability but their ink resistance to solvent type flexographic ink is still not enough.